The development of viruses as anticancer agents has been an intriguing yet elusive strategy. The goal of anticancer viral therapy is to inoculate a small percentage of tumor cells with replication competent viruses resulting in viral replication in the targeted tumor cells followed by cellular lysis (oncolysis) and infection of surrounding tumor cells. A key to viral oncolysis is genetic modification of the virus such that replication occurs principally in tumor cells and not in the surrounding normal tissue. Many strategies have focused on the use of genetically engineered viruses for oncolysis. For example, in one approach, attenuated retroviruses, modified to encode herpes simplex virus (HSV) thymidine kinase, were created to target dividing tumor cells [Culver, et al., Science 256:1550-1552 (1992); Ram, et al. Nat. Med. 3:1354-1361 (1997)]. In this technique, however, viral infection of tumor cells was limited since only 10 to 15% of tumor cells were actively progressing through the cell cycle. In another approach, conditional replication-competent adenoviruses (E1b deleted) were designed to replicate only in tumor cells lacking p53, however only 50% of tumors are estimated to contain nonfunctional p53 [Bischoff, et al., Science 274: 373-376 (1996); Heise, et al. Nat. Med. 3:639-645 (1997); Hollstein, et al., Science 253: 49-53 (1991)]. The success of these strategies, therefore has been limited experimentally only to small tumor xenografts.
Recently, genetically engineered replication-competent HSV has been proposed to treat malignant gliomas [Martuza, et al., Science 252:854-856 (1991)]. In anti-glioma therapy, HSV-1 mutants were constructed to preferentially replicate in proliferating tumor cells thereby eliminating the risk of widespread dissemination of the virus in the central nervous system, which is observed in rare cases of HSV encephalitis in human. Initial strategies focused on deletion of viral genes encoding enzymes required for viral DNA synthesis (e.g., thymidine kinase, ribonucleotide reductase [Martuza, et al., Science 252:854-856 (1991); Mineta, et al., Cancer Res. 54: 3963-3966 (1994)]. More recent studies centered on the use of HSV mutants that lack a newly identified γ134.5 gene involved in neurovirulence [Chou, et al., Science 250:1262-1266 (1990); Chou, et al., Proc. Natl. Acad. Sci. (USA) 89:3266-3270 (1992); Chou, et al., Proc. Natl. Acad. Sci. (USA) 92:10516-10520 (1995); Andreansky, et al. Cancer Res. 57:1502-1509 (1997)]. The combination of previous results suggested that a decrease in viral proliferative potential required for safe intracranial HSV inoculation, however, correlates with a decrease in the oncolytic potential of the virus [Advani, et al. Gene Ther. 5:160-165 (1998)]. The potential therapeutic effects of a genetically engineered HSV, having more potent antitumor efficacy than is possible for intracranial inoculation, has not been studied in models of common human tumors.
HSV offers many advantages as an oncolytic agent. The virus replicates well in a large variety on cancer cells and it destroys the cells in which it replicates. The virus can be attenuated by introducing specific deletions and it tolerates the insertion and expression of foreign genes [Meignier, et al., J. Infect. Dis. 158:602-614 (1988)]. Moreover, the functions of many HSV viral genes are known [Shih, et al., Proc. Natl. Acad. Sci. (USA) 81:5867-5870 (1984); Roizman, Proc. Natl. Acad. Sci. (USA) 93:113076-11312 (1996)]. The undesirable properties of HSV, however, include neuroinvasiveness, the ability to establish latency, and a capacity for reactivation from latent state.
Previous work has shown interactive effects of cytolytic capacity of modified HSV lacking both γ134.5 genes and ionizing radiation on glioma xenografts [Advani, et al. Gene Ther. 5:160-165 (1998)]. Ionizing radiation combined with inoculation with γ134.5-deficient HSV viruses resulted in supra-additive reduction in tumor xenograft volume and an enhancement in viral proliferation and intra-tumoral distribution in glioma xenografts.
R7020 is one such HSV strain attenuated by genetic engineering and tested in a variety of rodent, rabbit, and non-human primate models [Meignier, et al., J. Infect. Dis. 158: 602-614 (1988); Meignier, et al., J. Infect. Dis. 162:313-321 (1990)] which have shown that viral infectivity is attenuated in all species tested. A key property of interest in this strain is the lack of neuroinvasiveness even in the most susceptible species tested to date. R7020 is a modified HSV strain designed as a candidate for human immunization against HSV-1 and HSV-2 infections [Meignier, et al., Infect. Dis. 158: 602-614 (1988)]. Originally produced to be a live attenuated viral vaccine against HSV infection, R7020's has been examined for safety and stability in rodent and primate studies [Meignier, et al., J. Infect. Dis. 158: 602-614 (1988); Meignier, et al., J. Infect. Dis. 162:313-321 (1990)]. The construction of R7020 has been previously described [Meignier, et al., J. Infect. Dis. 158: 602-614 (1988); and Roizman, U.S. Pat. No. 4,859,587, incorporated herein by reference]. Briefly, wild-type HSV DNA consists of two regions of unique double-stranded DNA sequences flanked by inverted repeats [Roizman, et al., Proc Natl. Acad. Sci. (USA) 93:11307-11312 (1996)]. The inverted repeats regions contain two copies of five genes designated α0, α4, γ134.5, ORF P and ORF O. R7020 contains an HSV-2 DNA fragment inserted in place of one set of the repeats and therefore lacks only one of the two copies of the γ134.5 gene. Previously work has shown that, in certain cell lines, R7020 replicates more efficiently than viruses lacking both copies of the γ134.5 gene [Advani, et al. Gene Ther. 5:160-165 (1998)]. To date, R7020 has been subjected to limited trials in humans.
One of the causes of failure in cancer therapy is tumor cell resistance to conventional cytotoxic and/or hormonal treatments that arises from genetic instability caused by these agents and inherent instability of tumor cells. For example, p53 gene deletion or mutation may decrease tumor cell susceptibility to apoptosis induced by chemotherapy and/or radiation [Houldsworth, et al., Oncogene 16:2345-2349 (1998); Aas. et al. Nat. Med. 2: 811-814 (1998); Lowe, et al., Science 266:807-810 (1994); Dalta, et al., Cell Growth Differ. 6:363-370 (1995)] and mutations in the androgen receptor lead to hormone resistance in prostate cancer. Also, “gain of function” mutations, such as activation of the bc1-2 family of genes, enhances resistance to a variety of cytotoxic therapies. In addition to intrinsic genetic instability of tumor cells, commonly employed anticancer therapies that rely on DNA damage to tumor cells are mutagenic and a consequence of anticancer treatment is the selection and evolution of resistance to DNA damaging agents. One benefit of using viral lysis as an antitumor strategy is that viral lysis has the potential to overcome tumor resistance to conventional agents. Since tumor cell infection with replication component herpes results in cell lysis and is not per se mutagenic, selective evolution of tumor cells to evade herpes is less likely to occur within the tumor cell population.
Thus there exists a need in the art to identify and develop viral therapeutic agents and effective methods of treatment to retard and/or reduce tumor growth in patients in need thereof.